Separating iron from aqueous aluminum salt solutions



Feb. 25, 1964 H. LOEVENSTEiN 3,122,491

SEPARATING IRON FROM AQUEOUS ALUMINUM SALT SOLUTIONS Original Filed May24, 1955 INVENTOR. HIRSCH -LOEVENSTEIN,

5 QZZ7/ ATTORNEY United States Patent 3,122,491 SEPARATING l'RGN FRQMAQUEOUS ALUMWUM SALT SQLUTIGNS Hirsch Loevensteiu, The Dalles, 0reg.,assiguor to Harvey Aluminum Incorporated, a corporation of CaliforniaContinuation of application Ser. No. 510,762, May 24, 1955. Thisapplication Dec. 15, 1966, Ser. No. 76,066 3 Claims. (til. 204-412) Thisinvention relates to a process of separating iron from aqueous aluminumsulfate solutions. It pertains particularly to a process for recoveringexcess iron from aqueous aluminum sulfate solutions resulting from thedigestion of clay with sulfuric acid.

It has long been recognized that the alumina content of various widelydistributed and inexpensive clays may be converted to aluminum sulfateby digesting the clay with sulfuric acid. However, the application ofthis process on a large commercial scale for the production of aluminumsulfate which is sufficiently pure for many of its industrial uses hasbeen prevented heretofore by the fact that most of these clays, inaddition to containing the desired alumina, also contain a large amountof oxides of iron.

During the digestion of the clays, the iron oxides are converted by theaction of the sulfuric acid to ferrous sulfate, which contaminates thealuminum sulfate product unless steps are taken to remove it. Suchremoval is dihicult however, and, although several procedures have beensuggested for accomplishing it, up to the present time none has beenacceptable.

Thus it has been proposed to remove the iron from the raw aluminumsulfate solution by treating the latter with excess clay. However, thisdoes not remove a suliicient proportion of the iron and in additionrequires a preliminary exudation of any ferrous compounds present to theferric state, as well as substantial dilution of the solution, both ofthese factors adding materially to the cost of the operation. Stillfurther, part of the clay raw material is consumed in the iron removalstep and hence is lost to production, thereby adding a further elementof cost.

In another procedure, the raw aluminum sulfate solution is treated withaluminum trihydrate for the removal of iron. This eifectuates a moreefiicient removal of the iron than does the treatment with clay, but issubject to the other disadvantages which attend the clay treatment.Also, aluminum trihydrate is relatively costly and can not always beproduced at the plant in which the iron removal procedure is to bepracticed.

in still other proposed processes the iron is removed from the aluminum.ilfate liquor by the addition of various chemicals such as hydratedlime, potassium ferricyanide, potassium ferrocyanide, salts ofmanganese, etc., all of which have the common property of precipitatingthe iron from the solution. However, these methods are unsuitable forcommercial application because some of them require that the ferrousiron first be oxidized to the ferric state; the reagents themselves arevery costly; and principally, the reagent which has been used toprecipitate the iron is itself difficult to remove.

Accordingly it is the general object of the present invention to providea process for separating iron from the aluminum sulfate solutionsobtained by digesting high iron clays with sulfuric acid.

It is another object of this invention to provide a process forseparating iron, particularly excess iron, from aluminum sulfatesolutions, which process is applicable to solutions of low pH thuspermitting digestion of the clay to the point where nearly all of itscontent of alumina has dissolved.

It is another object of this invention to provide an electrolyticprocess for separating iron from aqueous aluminum sulfate solutionsusing a circulating iron-containing mercury electrode the iron contentof which is maintained within predetermined limits so that the electroderetains its mobility irrespective of the fact that its content ofamalgamated iron varies during the various stages of the process.

it is another object of this invention to provide an elec trolyticprocess for separating iron from aqueous aluminum sulfate solutionswhich is characterized by a comparatively high current efficiency.

It is another object of this invention to provide an electrolyticprocess for separating iron from aqueous alumi num sulfate solutionswhich makes use of a circulating mercury electrode, but which requires aminimum amount of this relatively expensive material.

It is another object of this invention to provide an electrolyticprocess for separating iron from aqueous aluminum sulfate solutionswhich may be operated at low temperatures without the externalapplication of heat.

The manner in which the foregoing and other objects of this inventionare accomplished will be apparent from the accompanying specificationand claims considered together with the drawing, the single figure ofwhich consists of a schematic apparatus flow sheet, partly in section,illustrating a system which may be used for the electrolytic separationof non from aluminum sulfate solutions in the manner described herein.

In general, the process of this invention comprises providing an aqueoussolution of a water soluble salt of aluminum, particularly aluminumsulfate, which contains at least 0.1% by weight of a water soluble saltof iron, particularly ferrous sulfate, calculated as Fe O This solutionis electrolyzed between a lead or other suitable anode and a mercurycathode containing at least 0.02 but less than 0.5% by weight metalliciron.

During the electrolysis, the concentration of the iron salt in thealuminum sulfate solution is not permitted to drop below the statedlevel of at least 0.1% by Weight, calculated as Fe O and the metalliciron content of the mercury cathode is not permitted to increase above alevel of 0.5% by weight. As the electrolysis proceeds, the ferrous ionspresent in the solution migrate to the mercury cathode and are releasedthere as metallic iron, which dissolves in or combines with the mercury.

The resulting solution of iron in mercury is processed for the recoveryof part of its iron content, precautions being taken to insure that theiron content of the mercury does not drop below 0.02% iron by weight.This may be accomplished by a reverse electrolytic procedure wherein theiron-mercury amalgam is made the anode of a suitable electrolytic cell.The electrolyzed mercury roduct, which still contains at least 0.02%metallic iron, then may be recirculated for use in the separation of afurther quantity of iron from a further quantity of raw aluminum saltsolution.

As has been indicated above, solutions of a variety of water solublealuminum salts containing iron salts as contaminants may be processed bythe presently described procedure. Suitable aluminum salt solutions thusinclude aqueous solutions of aluminum chloride, aluminum nitrate, and,particularly, aluminum sulfate. The solutions may be contaminated withiron in the form of its various water soluble salts, such as ironchloride, iron nitrate, or particularly iron sulfate.

The herein described procedure is particularly applicable, however, tothe sulfuric acid solution of aluminum sulfate and ferrous sulfate whichis obtained as a raw product from the digestion with sulfuric acid ofclays containing a high proportion, i.e. more than 5% by weight, ofiron, calculated as Fe O In the preparation of such solutions the clayafter crushing, calcining, and pulverizing is digested with sulfuricacid employed in amount sulficient to dissolve at least a majorproportion of the alumina content of the clay. In a typical case, thealuminum sulfate solution is substantially saturated with aluminumsulfate and may contain variable amounts of iron sulfate, in some caseswell above 2.0% by weight, calculated as Fe O Its pH is substantiallythat of an aqueous aluminum sulfate solution containing little or nofree sulfuric acid, i.e. from pH 0.3 to 3.0, usually about 2.0.

The foregoing or other suitable solution of Water soluble aluminum saltcontaminated with a water soluble iron salt is introduced into anelectrolytic cell having a construction suitable for the electrolytictreatment of such solutions. One or more of these cells may be employcd.Normally a number of iron removal cells may be combined in series withone iron recovery cell, in such a manner that the iron removed in theiron removal cells is nearly completely recovered in the iron recoverycell. The current efiiciency in the iron removal cells depends on theamount of iron initially and finally present in the solution. That ofthe iron recovery cell, however, approaches 100% in nearly all cases.

In the system schematically outlined in the drawing there are three suchcells 10, 12 and 14 for the separation of the iron from the solution anda fourth cell 16 for recovery of the iron as metallic iron. Cells 10, 12and 14 may have substantially identical constructions, that of cell 10being as follows:

The cell comprises an iron container 29, the sides of which preferablyare rubber covered or otherwise electrically insulated. It is filledwith an electrolyte comprising an aluminum sulfate solution 24.

An anode 26 is suspended in the electrolyte. It may be made of carbon,graphite or other material which is electrolytically inert to sulfatesolutions but preferably is made of lead. The anode is connected tocoupling 28 V which in turn is connected through line 39 to the positiveside of a supply of direct current 32 of sufiioient strength toestablish a cathodic current density of from to 100, preferably 15 to 40amperes per square foot of cathode surface.

The cathode 34 of the cell comprises a layer of mercury on the bottomthereof, spaced apart from the anode by a suitable distance. The mercuryhas a metallic iron content of at least 0.02% by weight, but not over0.5% by weight to insure its mobility.

Although a variable amount of mercury may be employed as the cathode, itis preferred to use a minimum quantity of this costly material and it isa feature of the present invention that only enough need be employed tocover the bottom of the cell, for example a layer about 5 mm. thick. Thecell then is provided with a motor driven agitator 36 which moves slowlyduring the period of electrolysis and agitates the mercury so that asthe iron is liberated it is absorbed efficiently and uniformly in thmercury, and any mercury hydride which may be formed is decomposed.

For continuous operation, the raw aluminum sulfate solution isintroduced continuously into cell 1% via conduit 3t; fed by pump '40from storage tank 42. Cathodic mercury is fed continuously to cell 1%via conduit 4- by mercury feed pump 45 from mercury sump 48. Partiallyused cathodic mercury is withdrawn continuously from cell 15 via conduit50 which includes a mercury continuity breaker 52, for interrupting thecircuit.

The partially electrolyzed solution overflows cell and is introducedinto the next cell 12 via conduit 54 while the cathodic mercury isintroduced thereinto via conduit Sit. Further electrolysis occurs inthis cell, the electrolyzed solution overflowing and being transferredvia conduit 56 into cell 14. The used cathodic mercury is introducedinto cell 14, via conduit 5'8 which includes mercury continuity breaker66.

As many iron removal cells of the type of cells 1t 12, 14 may beincluded in series circuit relationship to each other as may bedesirable or necessary. Thus cells it), 12 and 1 4 in the illustratedembodiment all are connected in series, current source 32 beingconnected through line 3t? to anode 26' in cell 10, cathode 34 of cell10 being connected through line 62 to the anode of cell 12; the cathodeof cell 12 being connected through line 64- to the anode of cell 14; andthe cathode of cell 14 being connected to the anode of cell 16.

As contrasted with iron removal cells 10, i2 and 14, cell 16 is an ironrecovery cell having for its function the recovery of the iron which hasbeen transferred by the electrolytic action of cells 19, 12 and 1-4-from the electrolyte contained therein to the circulating mercurycathode. Accordingly, the e-lectrolyzed aluminum salt solu-' tion, whichnow has a greatly reduced content of iron, is transferred via conduit 66to storage tank 6 3. The cathodic mercury, on the other hand, istransferred via conduit 70 directly into cell 16.

The construction or" cell 16 is somewhat analogous to that of cells it),12 and 14. It includes a receptacle 74 which is made of iron and linedwith an electrically in sulating material such as rubber. Contained inthe receptacle is a suitable electrolyte 76 which may be an aqueoussolution containing ferrous chloride and calcium chloride.

Suspended in the electrolyte is a cathode 73 which may be made ofcarbon, graphite, or other material, but which preferably is made ofiron. it is attached through connector 80 to line 8?. which in turn isconnected to the negative side of current source 32.

Cell 16 also contains a mercury electrode, which in this instance is theiron-containing mercury solution '84 introduced into the cell viaconduit 73*. This electrode is connected through line 86 to the cathodeof cell 14 and accordingly acts as the anode.

It is of sufiicient depth to cover the bottom of the cell and isagitated by the slowly moving motor driven agitator 38. It is withdrawncontinuously into conduit 90, which includes the mercury continuitybreaker 92 and which returns the mercury, now having a reduced contentof iron, to mercury sump d8, ready for cycling back into cell it) fortreatment of a further quantity of raw aluminum salt solution.

The electrolytic operations occurring in the iron removal cells may becarried out at relatively low temperatures, for example, at temperaturesCtf below 50 C. Acwrdingly, .no external heat need be applied to thesecells, the heat generated by the electrolysis being sufiicient toestablish a temperature suitable for eflicient operation. However ifdesirable or necessary for a given purpose, as

to accelerate the rate of reaction, the temperature of the electrolytein the cells may be increased to a higher level by the application ofheat from an external source.

80 C. to avoid rapid evaporation and overconcentration of the solution,resulting in precipitation of the aluminum sulfate. In the iron recoverycell a temperature of 6080 (3., obtained by external heating, ispreferred To summarize the operation of the herein described.

electrolytic system: I 7

Raw aluminum sulfate solution having a pH of between 0.3 and 3.0 whichpreferably is nearly saturated with aluminum sulfate and contains avariable amount of ferrous sulfate is pumped continuously from tank 42into electrolytic cell 19. There a current density is applied from DC.supply 32 which is sufiicient to separate part V of the iron as metalliciron at the circulating mercury cathode 3 2-. This deposition of iron isselective, the aluminum remaining in solution as aluminum sulfate.

'12 and thence by conduit 55 to cell Tn general, the temperature shouldnot be raised above about {from cell via conduit 50 to cell 12 andthence via conduit 58 to cell 14.

The electrolytic action occurring in the iron removal cells iscontrolled so that a major proportion of the iron is removed from theelectrolyte. The resulting solution of aluminum sulfate thus may besubstantially saturated with respect to aluminum sulfate. It may containa limited amount, e.g. from 0.1 to 0.3%, of iron calculated as F3203.

It is a particular feature of the presently described process, whenoperated in conjunction with a subsequent procedure for theprecipitation of the aluminum sulfate content of the solution by theaddition of sulfuric acid, that all of the iron need not be removed fromthe solution. This is in contrast to the procedures of the prior art inwhich iron is removed electrolytically from aluminum sulfate solutionsto such an extent that only traces of iron remain in the solution. italso is in marked contrast to the procedure of the prior art forobtaining aluminum sulfate from its solutions by evaporation techniques.

in the latter process, if the aluminum sulfate after concentration to asubstantial saturation contains 0.1% by weight iron calculated as Fe Oand is evaporated for the recovery of aluminum sulfate, the latterproduct will be contaminated with about 0.25% iron, calculated as 'Fe OThis iron content is so high as to make the aluminum sulfate unsuitablefor may of its applications.

However, when the instant process is employed together with a procedurefor recovering aluminum sulfate by precipitation with sulfuric acid,then the aluminum sulfate obtained from a solution contaminated with0.1% iron calculated as Fe O will contain less than 0.01% of thatmaterial and accordingly is well suited for all of its majorapplications.

Furthermore, the electrolytic operation is controlled so that thecirculating mercury cathode, which originally contained at least 0.02%by Weight metallic iron, does not at any time contain more than 0.5% byweight of that element. This is to insure its continued mobility so thatit may be transferred from cell to cell. if more than the indicatedamount of iron is present, then the resulting amalgarn may become stiffand sticky so that it will not flow readily, as is required for theherein described operation.

The electrolyzed aluminum sulfate solution in cell i4 is transferred vialine 66 to tank 63. The cathodic mercr' however, is trmsferred via line70 into the iron recovery cell if Here it is made the anode 84 of a cellincluding the iron cathode 7 3 and an electrolyte 76 comprising anaqueous solution containing an iron salt such as ferrous chloride, and,if desired, another salt such as calcium chloride.

The electrolytic action taking place in cell 16 is continuous, the ironcontent of the mercury entering the cell being reduced from a maximumvalue of about 0.5% to a minimum value of not less than 0.02% by weightwhen leaving the cell. it is critical to the success of the presentlydescribed operation that the latter residual content of iron bemaintained in the mercury. This is for the reason that if all of theiron is removed, then solution of the mercury content of anode 34occurs. This results in the deposition of mercury salts on the anodesurface; in migration of mercury to the cathode where it contaminatesthe deposit of electrolytic iron; and in consumption of part of themetallic mercury. These factors then ultimately will make impossible thesuccessful continuous operation of the process.

Where, however, the iron content of the mercury anode in cell 16 ismaintained at a level of at least 0.02% by weight, the mercury may beremoved continuously from the cell through conduit 3 into mercury sump43, whence it may be pumped by pump 46 through conduit 44 back into therst iron removal cell 10 for the processing of a further quantity of rawaluminum salt solution.

Also during the electrolysis of the aluminum salt solution theconditions are adjusted so that the concentration of iron in thesolution at no time drops below a value of 0.1% by weight, calculated asFe O This is for the reason that if more iron is removed the currentefiiciency drops rapidly and the electrolytic removal of the ironbecomes impractical.

The presently described process is illustrated further in the followingexamples wherein the proportions of the constituents are given inpercent by weight.

EXAMPLE 1 A solution of aluminum sulfate contaminated with ferroussulfate was introduced into the iron removal cell of an electrolyticsystem consisting of one iron removal cell such as is illustrated ascell 10 of the drawing and one iron recovery cell such as is illustratedas cell -16 of the drawing. The solution had been prepared by the digestion of a calcined, pulverized clay with 40% sulfuric acid. It containeda nearly saturated solution of aluminum sulfate and 0.83% ironcalculated as Fe O Its pH was 1.7.

The depth of the mercury in the two cells was 5 mm. A slowly movingagitator was employed for continuously stirring the mercury. The appliedcurrent density was 20 amperes per square foot of cathode surface andthe current efiiciency was 40.9%. The voltage of the iron removal cellwas 4.2 volts and that of the iron recovery cell 0.9 volt. Thetemperature of the iron removal cell was 14 C.

The two cells were operated continuously for two hours during which timethe iron content of the mercury entering the iron removal cell was 0.1%and that of the mercury leaving the iron removal cell was 0.34%. At theend of the operation, the aluminum sulfate solution contained but 0.34%iron calculated as Fe O the balance having been deposited on the ironcathode of the iron recovery cell.

EXAMPLE 2 A solution of aluminum sulfate contaminated with ferroussulfate and having a content and derivation substantially the same asthat employed in Example 1 was introduced continuously into anelectrolytic system consisting of three iron removal cells and one ironrecovery cell such as are illustrated in the drawing. The depth of themercury electrodes in the cells again was 5 mm. and the mercury wascontinuously agitated.

The applied current density was 20 amperes per square foot of cathodesurface; the current efficiency, 28.7%. The electrolysis was continuedfor 14 hours. The voltage and temperatures of the cells at the end ofthe operation were: Cell 1-5.0 volts and 19 C.; cell 24.7 volts and 20C.; cell 34.5 volts and 25 C.

The pH of the solution at the beginning of the operation was 1.6 and atthe end of the operation, 1.2. The solution entering the first ironremoval cell contained .79% iron calculated as H2 0 Upon leaving thethird iron removal cell it contained only 0.19% iron, calculated asF6203.

The iron content of the mercury entering the first iron removal cellvaried between 0.12% iron and 0.04% iron, analyses being made at 3 hourintervals. The iron content of the mercury leaving the third ironremoval cell varied from 0.30 to 0.44% Fe. This element therefore wasremoved efiectively in the iron removal cell to produce an aluminumsulfate solution having a concentration of iron sufiiciently low topermit the precipitation of substantially pure aluminum sulfate. It alsoproduced a mercury electrode material having an iron concentrationsuitable for reintroduction into the first of the series of iron removalcells.

EXAMPLE 3 A further quantity of aluminum sulfate solution having aderivation and properties substantially the same as those 'ciency. Ahigh current density may be employed.

set forth in Example 1 was introduced into a series of three ironremoval cells and one iron recovery cell substantially the same as aredepicted in the drawing.

This series of cells was operated continuously for six and one-halfdays. During the first 45 hours the cells were operated with a currentdensity of only amperes per square foot of cathode surface. The next 72hours the cells operated with a current density of amperes per squarefoot. During the last 39 hours they operated with a current density of31 amperes per square foot.

During this operation the iron recovery cell was disconnected from timeto time since the current efficiency of this cell was higher than thecombined current efficiency of the three iron removal cells. To controlthis cell, mercury samples were taken at the outlet and analyzed. Theiron content of the mercury then was permitted to increase to a value ofabout 0.05% before the cell was connected and to decrease to a value ofabout 0.02% before the cell was disconnected.

Table I Cathodic current density 10 Amp./ 20 Amp./ 31 AmpJ sq. ft. sq.ft. sq. ft.

Current cihciency .perceut. a 23. 4 24. 4 21. 5 Iron content of aluminumsulfate solution entering cell 1 1 percent 78 82 82 Iron content of alunnnurn sulfate solution leaving cell 3 1 percent" 185 .24 .28 Ironcontent of mercury entering cell 1 (average) 2 percent .07 11 06 Ironcontent of mercury leaving cell 3 (average) 2 ttpereentu .36 .42 32Terminal voltage:

19 26 19 29 33 19 36 Terminal pH- 7 5 .4

1 Calculated as F8203.

2 Calculated as Fe.

The foregoing results indicate clearly the effectiveness of the hereindescribed procedure in separating iron from aluminum sulfate solutionsby an electrolytic process operated at remarkably low temperatures andusing an electrolyte having a pH as low as 0.4.

Accordingly it will be apparent that by the present invention I haveprovided a process for the separation of iron from solutions of aluminumsalts contaminated with iron salts which is attended by severaladvantages of the greatest significance. It is applicable to solutionsof low pH. It is operable with a high degree of current efli- It makesuse of a minimum amount of costly mercury. It may be operated at lowtemperatures without the application of external heat. It separates theiron elliciently, giving an aluminum sulfate product from which thealuminum sulfate may be precipitated in substantially pure form by thesubsequent addition of sulfuric acid. Accordingly, it makes feasible aprocedure for the commercial recovery of substantially pure aluminumsulfate from cheap, abundant clay raw materials containing as con- 1.The continuous process of separating iron from aqueous aluminum sulfatesolutions which comprises providing an aqueous solution containingaluminum sulfate and ferrous sulfate which is substantially saturatedwith aluminum sulfate and which has a pH of 0.3 to 3.0, passing thesolution through a series of iron removal electrolytic cells each ofwhich contains an anode, passing a mercury cathode material containingfrom 0.02 to 0.5% by weight metallic iron through the series of ironremoval cells, electrolyzing the solution in the iron removal cellswhile maintaining the concentration of ferrous sulfate in the solutionat a level of about 0.1% to 0.3% by weight and the metallic iron contentof the mercury cathode material at a level of not less than 0.02% normore than 0.5%, thereby causing the separation of iron from'the solutionand its amalgamation with the mercury cathode, electrolyzing theresulting amalgam of metallic iron and mercury in an iron recoveryelectrolytic cell, using the amalgam as the anode, until its ironcontent has been reduced to a level not less than 0.02% by weight,cycling the resulting mercury cathode material of reduced iron contentback to the electrolysis of a further quantity of aluminum sulfatesolution, and following electrolysis, subjecting the aluminum sulfatesolution to the action of sulphuric acid to precipitate aluminumsulfate.

2. The process of separating iron from aqueous solutions of aluminumsalts which comprises providing a low pH aqueous solution of a Watersoluble aluminum salt containing a contaminating amount of a watersoluble salt of iron, electrolyzing the aluminum salt solution using acathode comprising mercury containing at least 0.02 but less than 0.5%by weight metallic iron, whereby to transfer iron from the aluminum saltsolution to the mercury cathode, maintaining the electrolysis until theiron salt in the aluminum salt solution is reduced to from about 0.1% toabout 0.3% by weight, during the electrolysis maintaining in the mercurycathode at least 0.02% but less than 0.5% by weight metallic iron bycontinuously removing mercury from the electrolysis, electrolyzing themercury using the latter as the anode to remove therefrom the ironcontent in excess of said maintained amount, and returning the mercuryto the aluminum salt electrolysis and following electrolysis, subjectingthe aluminum salt solution to the action of sulfuric acid to precipitatealuminum sulfate.

3. The process of claim 2 wherein the aluminum salt solution is aluminumsulfate solution having a pH of 0.3

Palmaer Mar. 14, 1933 Palmaer Aug. 21, 1934 Schumacher et al Nov. 27,1945'

1. THE CONTINUOUS PROCESS OF SEPARATING IRON FROM AQUEOUS ALUMINUMSULFATE SOLUTIONS WHICH COMPRISES PROVIDING AN AQUEOUS SOLUTIONCONTAINING ALUMINUM SULFATE AND FERROUS SULFATE WHICH IS SUBSTANTIALLYSATURATED WITH ALUMINUM SULFATE AND WHICH HAS A PH OF 0.3 TO 3.0,PASSING THE SOLUTION THROUGH A SERIES OF IRON REMOVAL ELECTROLYTIC CELLSEACH OF WHICH CONTAINS AN ANODE, PASSING A MERCURY CATHODE MATERIALCONTAINING FROM 0.02 TO 0.5% BY WEIGHT METALLIC IRON THROUGH THE SERIESOF IRON REMOVAL CELLS, ELECTROLYZING THE SOLUTION IN THE IRON REMOVALCELLS WHILE MAINTAINING THE CONCENTRATION OF FERROUS SULFATE IN THESOLUTION AT A LEVEL OF ABOUT 0.1% TO 0.3% BY WEIGHT AND THE METALLICIRON CONTENT OF THE MERCURY CATHODE MATERIAL AT A LEVEL OF NOT LESS THAN0.02% NOR MORE THAN 0.5%, THEREBY CAUSING THE SEPARATION OF IRON FROMTHE SOLUTION AND ITS AMALGAMATION WITH THE MERCURY CATHODE,ELECTROLYZING THE RESULTING AMALGAM OF METALLIC IRON AND MERCURY IN ANIRON RECOVERY ELECTROLYTIC CELL, USING THE AMALGAM AS THE ANODE, UNTILITS IRON CONTENT HAS BEEN REDUCED TO A LEVEL NOT LESS THAN 0.02% BYWEIGHT, CYCLING THE RESULTING MERCURY CATHODE MATERIAL OF REDUCED IRONCONTENT BACK TO THE ELECTROLYSIS OF A FURTHER QUANTITY OF ALUMINUMSULFATE SOLUTION, AND FOLLOWING ELECTROLYSIS, SUBJECTING THE ALUMINUMSULFATE SOLUTION TO THE ACTION OF SULPHURIC ACID TO PRECIPITATE ALUMINUMSULFATE.